The lock mechanism comprises a first subunit (6) arranged to be fixed on an outer face of an upright tubular member (7) of the hinged leaf and a second subunit (8) arranged to be fixed within the tubular member (7). The second subunit (8) comprises a frame (9), a bolt member (10) movably mounted on said frame (9) between a projected and a retracted position, and a bolt operating mechanism which comprises a gear train (20, 21, 22) to increase the travel of the bolt member (10). The first subunit (6) comprises operator means (26, 28, 29). The lock mechanism further comprises a rotary shaft (32) by means of which the bolt operating mechanism can be coupled to the operator means so that only a hole has to be made in the tubular member (7) of the leaf. The operator means comprise a hinged lever (26) movable between a pushed in position and a pulled out position, the bolt member (10) moving from its projected to its retracted position when the lever (26) is moved from its pushed in to its pulled out position and vice versa. In this way, the bolt member (10) is locked in its projected position, wherein it extends in a reception hole in the ground, when the double door or gate is closed.